A printing press is basically a sequence of a series of nips, created at the contact of opposing rotating cylinders, with voids on the printing plate to insure no ink will be deposited on the substrate where it is not wanted. The number of nips in a printing press determines the simplicity/complexity of the printing process. The surface characteristics of the applicator(s) of the ink from one surface to another in the nip(s) determines how much of the ink gets transferred from and how much remains on the applicator. Seldom, if ever, does all the ink on an applicator get transferred to its recipient surface at the points of contact in the nip. In the printing trade, this is known as the ink film split in the nip, or simply ink film split, or film split.
The four major printing processes transfer ink—typically a pigmented liquid capable of becoming a colored film on a substrate when dry—in paste or fluid consistency, from an applicator, known as the printing plate, securely mounted on a rotatable cylinder, onto another surface in a nip created with another, opposing, rotatable cylinder. The ultimate deposition of the ink, directly or indirectly, is onto a substrate—paper, paperboard, plastic, plastic coated paper/paperboard, foil-laminated paper/paperboard, vacuum-metalized paper/paperboard, etc.—held securely on a rotatable, opposing cylinder, known as the impression cylinder, in a nip.
I. In letterpress printing, the original printing process pioneered by Johann Gutenberg in Germany in the 16th Century, the printing plate is made of a single layer of metal with ink-containing projections which transfer the ink directly onto the substrate which forms the final product from the raised portions of the printing plate.
Typically, letterpress ink is a paste-type with little or no solvent. In a sheet fed operation, there is no auxiliary inter-station drying equipment. Thus, in multi-color letter presses, the various colors of the ink are still wet when the substrate arrives at the next print station from the previous print station. Frequently, the ink on the sheets in the delivery pile is still not dry, so some holding of piles (lifts) after they have been taken off the press is necessary before the sheets thereon can go to the next operation for further processing into a finished product.
II. In flexographic printing, known as flexography, the printing plate also transfers the ink on it directly onto the substrate, also forming the final product, from raised portions thereof, typically molded rubber or plastic.
In flexo, the printing plate is typically a single layer of a plastic material with ink-receiving projections. Typically the ink is fluid. That is, it contains a significant amount of solvent, which does not become part of the dried ink film. Typically, the amount of solvent is more than the ink vehicle and pigment combined, often two or three times as much. Originally, and still mainly today, the solvent is a volatile organic compound (VOC), although water is now used instead of solvent on some substrates, primarily paper and paperboard. Typically, this solvent or water is driven off the substrate in commercial and folding carton printing by interstation and end of press auxiliary drying equipment. Thus, flexo ink typically is dry when the substrate arrives at the next print station from the previous print station. Typically, there is no auxiliary drying equipment interstation or end of printing press on corrugated paperboard substrates, because its print surface is quite porous. Here, a substantial amount of the ink sinks beneath the surface. Thus, here the ink dries by the combination of absorption and ambient solvent evaporation. Whatever the surface, however porous or non-porous it may be, the solvent or water (which holds the ink vehicle and pigment in suspension—i.e., in emulsion) does not become part of the dry ink film on the substrate.
Because all the colors of flexo ink are dry when the printing is finished, the printed substrate is ready to go to the next operation without holding. Indeed, because all the ink is dry, subsequent operations on the substrate, such as embossing, perforating, cutting, creasing, folding, gluing, binding, can be done in-line on the printing press. In-lining has economic advantages.
In both letterpress and flexo printing, there are a total of two nips: (1) ink from a fountain roller or rollers onto the raised portion of the printing plate; (2) ink from the raised portions of the printing plate onto the substrate, in a nip with the impression cylinder.
III. Rotogravure printing—gravure for short—like letterpress and flexo—transfers the ink from every print station directly onto the substrate. However, in contrast to letterpress and flexo, gravure ink is deposited directly onto the substrate from depressions in the printing plate, or directly from the printing cylinder, typically created by an etching (engraving) process. These depressions are called cells. The cubic capacity of the cells determines the amount of ink therein, and the amount of ink deposited on the substrate. As in letterpress and flexo, there are a total of two nips, ink roller(s) to plate on the plate cylinder, plate to substrate on the impression cylinder.
Typically, the ink in gravure printing is fluid, as in flexo, typically with a high percentage of solvent, which typically is driven off the substrate by auxiliary interstation and end of press drying equipment. Thus, as in flexo, no holding is necessary before the substrate can go the next operation. Thus, as in flexo, in-lining for subsequent operations is possible.
As in letterpress and flexo, typically ink does not cover the entire form. There are some voids, because typically the printed form has some voids. As in letterpress and flexo, there are a total of two nips.
IV. Lithographic (offset) printing, unlike letterpress and flexo or gravure, utilizes a planar printing plate, i.e. one that has no raised or recessed surfaces. This 1-level printing plate distinguishes it from all other print processes and creates complexities and issues to contend with, not inherent in all other print processes.
One consequence of the planar litho printing plate is that currently litho printing is indirect—in contrast to the foregoing 3-print processes. Thus, the planar litho plate transfers the ink that is on it onto a composite rubber/fabric material called a blanket, mounted securely on a separate cylinder, called the blanket cylinder, in its own nip. It is this rubber/fabric blanket, which then transfers the ink on it onto the substrate securely held on the impression cylinder, in another nip. Thus, the derivation of the word offset to describe the litho process and printing press.
How is the paste-type litho ink kept off the voids where ink is not wanted/needed? The typical, rather thin (compared to letterpress) metal printing plate in litho printing, is chemically treated in the image areas to make it water-repellant in a department of the printing plant called pre-press, or more commonly, the plate room. This chemical treatment of the image areas of the metal planar plate is designed to keep water off the image areas. Keeping ink off the non-image (i.e., void) areas is achieved by a water-based fountain solution—typically water with a wetting agent/agents, whose purpose is to make water wetter—which is applied to the planar printing plate from a fountain on a print station in its own nip, prior to the application of ink from its fountain onto the printing plate in its own nip. It wets the non-imaged areas (to keep ink off), but not the imaged areas.
The indirect transfer of the litho ink on the planar printing plate, and the introduction of a water-based fountain solution onto the printing plate prior to the deposition of ink thereon, makes the current litho process more complex and difficult in maintaining print fidelity and color consistency throughout a run, than the other three major print processes.
In the current planar litho printing plate process, with its indirect transfer of ink onto the substrate, there are four nips versus two for the other three print processes. It is a truism, the more nips in a print process, the more chances for something to go awry and the more press-intensive it is for the press crew to manage the operation to consistently produce saleable product with least waste and press downtime, with least chance of rejection by the customer for color variation.
In the litho industry, the color variation and deviation from spec and initial color okay is attributed to ink emulsification, or ink/water imbalance. It is the pressman's nightmare. It causes him to go hopping around 4-5-6-7-8-9-10 printing press stations to make a move or moves on the various print stations to bring the ink and fountain solution back into balance on each print station.
The color variation from spec and color okay at the beginning of the run is the major cause of waste and press downtime on whatever the substrate, on whatever make or size of litho press employing planar printing plates.